MC 2205 meters respond unusually

Deth

Young Collector

• Oct 9, 2014

• #1

Currently working on an MC 2205 and the meters act very strange. When i have one channel of the amp going the meter for the channel works, but the other channel's meter goes below zero. This problem was not present until i removed both boards from the front of the chassis and corrected some bad solder joints, then reassembling it. When i turn both channels to equal volume and the meters remain at zero. What is causing this problem? does anyone know?

 

petehall347

the brandy coffee man

• Oct 9, 2014

• #2

sounds like you inadvertently cause a short .use a magnifying glass to check over your work .could just be a tiny whisker of solder .

 

clinic-audio

all on YAMAHA untill 1990

• Oct 10, 2014

• #3

MC2205 meters respond unusually

Hello Deth !

What is causing this problem? does anyone know?

1) do you mean that you can hear music in speakers and meterS do not work at all ?

2) does the speaker switch (-20dB to hold ) does affect this phenomenon ?

 

Deth

Young Collector

• Oct 10, 2014

• #4

The meters do work, it is hard to explain. If i have both channels running at the same time the meters do absolutely nothing, but if i use 1 channel at a time the meters work as they should.

 

LexDM3

AK Subscriber

Subscriber

• Oct 10, 2014

• #5

Any chance that something is either shorted or connected backwards? It sounds like one meter drive circuit is cancelling out the other when both have signal, similar to having little or no bass because the speakers are connected out of phase.

 

Classic_CJ

Well-Known Member

• Oct 10, 2014

• #6

LexDM3 said:

Any chance that something is either shorted or connected backwards? It sounds like one meter drive circuit is cancelling out the other when both have signal, similar to having little or no bass because the speakers are connected out of phase.

Click to expand...

Yep. That sounds about right. Re-check those solder connections, especially for bridging (cross-connecting) some foils or pads. Also, a lifted ground connection for the two meters would place the meter deflection coils in series with one another, and if so I'd guess it would do exactly what you're describing.

 

LexDM3

AK Subscriber

Subscriber

• Oct 10, 2014

• #7

Classic_CJ said:

Also, a lifted ground connection for the two meters would place the meter deflection coils in series with one another, and if so I'd guess it would do exactly what you're describing.

Click to expand...

That's a real possibility and agrees with the symptoms you're seeing. Measure the meter ground leads with a meter and see if they're really grounded or just floating. Have you ever followed a car that has its tail, brake and signal lights working strangely so that the tail goes off when the brake light goes on? That's often a disconnected ground.

 

Deth

Young Collector

• Oct 10, 2014

• #8

I'll be checking the ribbon cable connections on the board, i think i know what i did wrong but ill be keeping you guys posted.

 

sKiZo

Hates received: 92644 43.20°N 85.50°W

• Oct 11, 2014

• #9

Known problem.

There's some threaded posts inside the meter that the signal wires hook to. Those corrode and either go open or intermittent. Bit of work getting the backs off, but that usually corrects any problems.

Do you also get the dead needle thing? Turn it on, and no movement, flick the speaker on/off and it starts working. Same thing.

Big writeup on it somewhere around here ... aha!

http://www.audiokarma.org/forums/showpost.php?p=8057218&postcount=7

 

Classic_CJ

Well-Known Member

• Oct 11, 2014

• #10

Posted by sKiZo ---

*** Known problem. There's some threaded posts inside the meter that the signal wires hook to. Those corrode and either go open or intermittent. Bit of work getting the backs off, but that usually corrects any problems. ***

You're correct about this issue with the oxidation inside the meter terminals, but it's unlikely to explain the specifics of what the OP noted in his description, which is that the meters work fine if he only applies signal to one channel at a time, but when he applies signal to both, they stop working.

As LexDM3 noted, the problem sounds like the stereo signals are cancelling each other, and I agreed, with a lifted meter common/ground being the most probable cause. It's much easier to check for that possibility first before taking the meters apart-- a tricky job, as you noted.

 

Deth

Young Collector

• Oct 19, 2014

• #11

I believe i found the problem, and hopefully any 2205 owners in the future will see this. Basically the ribbon cable that connects two boards in the front comes undone and the connections become intermittent, best way to fix this is with solid core wire. I'll be fixing it soon and ill give you guys an update.